For example, a protection relay that protects a power system detects a system accident based on the magnitudes of an input current and input voltage, phase relationship and the like and performs a protection control operation to protect the power system. In this case, the state of the external device is input by use of a contact input signal and, for example, contact input (Binary input) is detected according to the magnitude of an external input voltage value with respect to a preset threshold voltage.
The conventional contact input circuit realizes detection or non-detection of contact input by adjusting a current value flowing through a photocoupler and setting a threshold voltage by means of resistors connected in series with or in parallel with the photocouplers as a current-limiting resistor system. In the respective contact input circuits, one threshold voltage is set for one circuit. Therefore, in order to cope with a plurality of threshold voltages, it is necessary to change the setting of a threshold voltage with respect to an external input voltage. Therefore, as a general means of coping with this, a circuit having resistance constants changed to change the setting of a threshold voltage is prepared and a circuit board thereof is mounted.
In the contact input circuit of the current-limiting resistor system, there is provided an input circuit of a programmable controller that can vary the duty ratio to reduce heat generation by ON/OFF controlling output of a photocoupler that transmits a detected contact input signal to an operation unit by means of a switching element (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2008-152309 (that is hereinafter referred to as “Patent Document 1”)). Further, a constant current circuit is realized by making an attempt to reduce heat generation of a contact input circuit by use of a bypass current circuit and setting two transistors in an ON state by use of a voltage value defined based on a resistance-divided voltage and Zener diode (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2005-101781 (that is hereinafter referred to as “Patent Document 2”)).
However, in the contact input circuit based on the conventional current-limiting resistor system, the power consumption in the resistors serially connected for adjustment of the threshold voltage becomes larger and becomes a cause of a temperature rise of the device. Further, since the forward current value that causes the photocoupler to emit light is adjusted by means of resistors and depends on the performance of the photocoupler, it becomes difficult to adjust the threshold voltage for wide-range input voltages by use of one contact input circuit. Therefore, if an attempt is made to set a plurality of threshold voltages in a wide range by use of one contact input circuit, the threshold voltage adjustment cannot be achieved simply by changing the resistance constant.
Further, in Patent Document 1, since a switching element is turned ON only when photocoupler output is read, heat generation due to the current-limiting resistor can be reduced, but it is difficult to adjust the threshold voltage for wide-range input voltages by use of one contact input circuit. Further, in Patent Document 2, since a bypass current circuit is provided, the circuit configuration becomes complicated although the threshold voltage can be adjusted for wide-range input voltages by use of one contact input circuit.
Under the circumstances, it is desired to provide a protection relay including an input circuit capable of adjusting a threshold voltage for wide-range input voltages by use of one contact input circuit and reducing a heat generation amount without making the circuit configuration complicated.